Trivial fixes in XML comments

And added an #if around using in RequestParameters.cs (worth having a
look, may have implications when building on different platforms)

Octokit/Models/Request/RequestParameters.cs

@@ -1,5 +1,7 @@
 using System;
+#if !PORTABLE
 using System.Collections.Concurrent;
+#endif
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;